Conventional low density polyethylene (LDPE) is noted to have a desired balance of properties, which include good processability as defined by low motor load, minimal neck-in, which results in a reduction of waste due to edge trim, and the ability to be drawn to very low thicknesses at high line speeds. When used in extrusion coating and extrusion lamination applications, the desired level of adhesion is a requirement to meet the final package/article end use application. For most paper and paperboard applications, like gable top milk cartons, drink cups, sugar pouch and the like, having the ability to achieve fiber tear adhesion, at high line speeds, and/or fiber tear adhesion, at lowered coating/lamination temperatures, are desired.
Companies that extrusion coat a variety of polymers, including low density polyethylene (LDPE), onto paper and paperboard, continually have problems obtaining good “fiber tear” adhesion, which is the amount of adhesion required, to have an adhesion between the polymer to the paper greater than the adhesion of the paper fibers to themselves. When the polymer is pulled from a paper substrate, if fiber tear occurs, this adhesion is considered acceptable. If the polymer is pulled from the paper without fibers (without fiber tear), this is considered unacceptable adhesion. Improved adhesion is also desired in the extrusion lamination process, which is used to manufacture multilayer substrates. Ethylene-based polymers used for coatings and films are disclosed in the following references: International Publication Nos. WO 2011/071843, WO 1991/18944; U.S. Pat. Nos. 5,178,960, 3,860,538, 4,714,741, 6,558,809, 4,962,164, 3,676,401; GB 1448062; EP 0230143B1; Ward et al., Ethylene-Carbon Monoxide Extrudable Adhesive Copolymers for Polyvinylidene Chloride, June 1988 Tappi Journal, pp. 140-144; Scott et al., Degradable Polymers, Principles and Applications, Chapter 8: Ethylene-carbon monoxide copolymers, pp. 156-168, Chapman and Hall (1995). See also WO2014/105608, and U.S. Provisional 62/018,014 filed Jun. 27, 2014.
U.S. Pat. No. 6,407,191 discloses the use of carbonyl group containing chain transfer agents to obtain improved polymer processing and performance properties in extrusion processes and applications. The level of carbonyl group containing compound (ketones and aldehydes are claimed) ranges from 0.1 to 0.5 wt %. Production of high molecular weight polymers will restrict the level of ketonic or aldehydic chain transfer agents to be used to control the melt-index, and therefore the maximum level which can be incorporated in the polymer to 0.5 wt %.
It is a well known that, when running extrusion coating lines, at higher and higher line speeds, with conventional polymers, unacceptable fiber tear adhesion results. This adhesion is related to the oxidation of the molten polymer, as it exits the extrusion die, and before it comes in contact with the paper (or the time in the air gap). At higher line speeds, the molten polymer curtain has less time to oxidize, which can reduce adhesion. There is a need for new ethylene-based polymers that can be used in high speed extrusion coating lines, and which have good adhesion to paper substrates, and which can be used at lower melt temperatures, and/or at higher line speeds of the paper substrate. The polar nature of ECO copolymers provides improved adhesion over traditional LDPE for polar substrates, like paper, paperboard, foil, and metalized films.
International Publication No. WO 2013/059042 describes using fresh ethylene and/or CTA (chain transfer agent) feed distributions to broaden MWD and increase melt strength, while remaining process conditions are constant. International Publication No. WO 2013/078018 describes low density ethylene-based polymers with broad MWDs and low extractables, made in a tubular reactor, in the absence of an added crosslinking agent and/or comonomer with crosslinking capability.
International Publication WO 2013/078224 describes broad MWD, tubular LDPE resins with low extractables. The polymerization conditions need to be carefully selected and balanced to reduce extraction at higher molecular weights. Important process parameters include maximum polymerization temperatures, reactor pressure, and the type, level and distribution of the CTA.
There remains a need for new ethylene-based polymers that have improved adhesion to substrates. There is a further need for such polymers that can be made in a tubular reactor at low gel levels. There is a further need for such polymers that also have lower n-hexane extractable content. These needs have been met by the following invention.